Spontaneous Phosphorus‐Halogen Bond Cleavage in N‐Heterocyclic Halogenophosphanes Revisited: The Case of P–Br and P–I Bonds

P‐bromo‐ and P‐Iodo‐substituted N‐heterocyclic phosphanes (NHP) were synthesized by halogen exchange starting from the P‐chloro compound and characterized by spectroscopic data and X‐ray diffraction studies. Whereas the Br‐NHP still forms a molecular crystal, the solid‐state structure of the I‐derivative consists of ion‐pairs which assemble via secondary P···I interactions to form one‐dimensional coordination polymers. Computational studies indicate that the bond dissociation is due to a combination of intramolecular stabilisation (aromatisation of the phosphenium cation formed) and intermolecular Lewis‐acid/base interactions. Reaction of the I‐NHP with I₂ proceeded under complete P–I bond cleavage to give an ionic phosphenium triiodide whose single‐crystal X‐ray diffraction study revealed a unique supramolecular structure.     

Structural and magnetic properties of layered Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films

Layered cobalt oxides Ca₃Co₄O₉ thin films have been grown directly on c-cut sapphire substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy characterizations show that the deposited films present the expected monoclinic structure and a texture along the direction perpendicular to the Al₂O₃(001) plane. The Ca₃Co₄O₉ structure presents six variants in the film plane. Rutherford backscattering spectroscopy shows that the films are stoichiometric and that the film thickness agrees with the nominal value. The susceptibility χ of the films, recorded along the c-axis of the substrate, after field cooling and zero field cooling in an applied field of 1 kOe shows two magnetic transitions at 19 and 370 K which agree well with previous findings on single crystal samples. In turn, at low temperature (5 K), the magnetization curve along the c-axis exhibits coercive field and remanent magnetization much smaller than those reported for bulk samples, which can be related to the influence of structural variants and structural defects.     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001     

Solving the triangular Ising antiferromagnet by simple mean field

Few years ago, application of the mean field Bethe scheme on a given system was shown to produce a systematic change of the system intrinsic symmetry. For instance, once applied on a ferromagnet, individual spins are no more equivalent. Accordingly a new loopwise mean field theory was designed to both go beyond the one site Weiss approach and yet preserve the initial Hamitonian symmetry. This loopwise scheme is applied here to solve the triangular antiferromagnetic Ising model. It is found to yield Wannier's exact result of no ordering at non-zero temperature. No adjustable parameter is used. Simultaneously a non-zero critical temperature is obtained for the triangular Ising ferromagnet. This simple mean field scheme opens a new way to tackle random systems. Received 14 November 2001 / Received in final form 22 March 2002 Published online 19 July 2002     

光固化含氟聚合物的合成进展

综述了自由基光固化和阳离子光固化含氟聚合物的合成进展,并对其进行了讨论.     

Self-association behaviour of alcohols in diluted aqueous solutions

Summary Our recent studies related to the properties of alcohol/water mixtures show the occurrence of some kind of molecular aggregation in the water-rich region of composition beyond a threshold alcohol concentrationx ₂ ^*. The observed behaviour suggests that forx ₂<x ₂ ^* the alcohol molecules are essentially dispersed and surrounded by ?water cages? where the short-range order and microdynamic of water molecules are changed with respect to those of pure water. Alcohol molecules are in mutual contact at higher concentration only when almost all water is involved in hydration shells of alcohol molecules. The structural transition atx ₂ ^* resembles, for some aspect, the micellization process. The main results of these investigations are reviewed and discussed in this paper. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8 1994     

microscopic structure of the solid-liquid interface of aluminum

Summary A simple quasi-static model is used to calculate the potential energy field for a probe Al ion sitting in the liquid phase in the neighbourhood of the melt-crystal interface in aluminum at 933 K. Calculations make use of anab initio potential, already described in the literature, which well reproduces several bulk properties. The calculated field exhibits a complex structure; a relevant feature of it consists in the appearance in the liquid of potential minima which correspond either to substitutional or to interstitial positions in the growing lattice.

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Riassunto Il campo di potenziale a cui è sottoposto uno ione di alluminio nella regione liquida vicina all’interfaccia solido-liquido di Al a 933 K è stato calcolato usando un semplice modello microscopico quasi statico. Il calcolo si basa su un potenziale calcolato da principi primi, già descritto in letteratura, che riproduce bene diverse proprietà del liquido e del solido. Il risultato principale è l’esistenza di un campo di potenziale strutturato nel liquido vicino all’interfaccia. Questo campo presenta dei minimi che corrispondono a posizioni sostituzionali od interstiziali nella regione di crescita del reticolo.

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Резюме Предлагается простая квазистатическая модель для вычисления поля потенциальной энергии для зондирующего Al иона, находящегося в жипкой фазе вблизи границы плавящегося кристалла Al при 933 К. При вычислениях используется потенциал, уже описанный в литературе, который хорошо воспроизводит некоторые объемные свойства. Вычисленное поле имеет сложную структуру, характерной особнностью которой является появление в жидкости потенциальных минимумов, которые соответствуют либо положению внедрения или положению замещения в растущей решетке.

     

Molecular structure of carbonyl fluoride as studied by gas electron diffraction and microwave data

The molecular structure of carbonyl fluoride has been determined by electron diffraction. The results have been used in conjunction with the rotational constants reported by Carpenter in a combined structure analysis. The values so obtained are r_z (C=O) = 1.1717 ± 0.0013 Å, r_z (C-F) = 1.3157 ± 0.0005 Å, and ∠_zF-C-F = 107.71 ± 0.08°. These agree with the corresponding parameters estimated by Carpenter from the rotational constants alone. The effective constants, α₃, representing the cubic anharmonicity of bond stretching vibrations have been estimated.     

Magnetoelastic behaviour of amorphous ferromagnetic ribbons in response to an applied tensile stress

Summary The magnetization changes, ΔM, induced by an external tensile stress, σ, in metallic-glass ribbons are theoretically calculated starting from the simplified model generally used for describing the magnetoelastic coupling in amorphous metals. Taking into account the discrepancies between the ideal behaviour and the real one as revealed in commercial amorphous ribbons, we show that the experimental results, concerning ΔM as a function of σ, can be well predicted by the theoretical evaluations. On the other hand, it results that a suitable ferromagnetic ribbon can be a magnetoelastic sensor able to measure variations of ≃1 MPa up to the 100 MPa stress.

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Riassunto Le variazioni di magnetizzazione prodotte da una tensione esterna in nastri di metallo amorfo sono state teoricamente calcolate partendo dal modello semplificato generalmente adottato per descrivere l'accoppiamento magnetoelastico in detti materiali. Tenendo conto delle differenze tra il comportamento reale e quello ideale, si mostra come i risultati sperimentali, ottenuti nei nastri amorfi in commercio, possono essere ben previsti mediante le valutazioni teoriche. D'altra parte, si mostra come un nastro amorfo, opportunamente selezionato, possa rilevare variazioni di tensione di 1 MPa sino a valori base della sollecitazione di circa 100 MPa.

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Резюме Теоретически вычисляются изменения намагниченности, ΔM, индуцированные внешним растятивающим напряжением, σ, в металлических аморфных полосах, исходя из упрощенной модели, обычно используемой для описания магнитоупругой связи в аморфных металлах. Учитывая различия между идеальным поведением и реальным поведением, мы показываем, что экснериментальные результаты в коммерческих аморфных полосах для зависимости ΔM от σ могут быть получены из теоретических вычислений. С другой стороны, отмечается, что соответствующая ферромгнитная полоса может представлять магнитноупругий датчик, способный измерять изменения напряжения от ≃1 МПа вплоть до 100 МПа.